Magnetic cores and methods of making the same



De- 23, 1951 A. HARENDzA-HARINXMA 3,04g825 MAGNETIC CORES AND METHODS OF MAKING THE SAME Filed Dec. 3, 1959 2 Sheets-Sheet 1 fF//f/z/ y\ N s M) s Dec. 26, 1961 A. HARENDzA-HARINXMA 3,014,825

MAGNETIC CORES AND METHODS OF' MAKING THE SAME Filed Dec. 3, 1959 2 Sheets-Sheet 2 //l/ VA/7.0@

3,tl14,$25 MAGNETIC CRES AND METHODS F MAKING v THE SAME Alfred Harendza-Harinxma, Chicago, Ill., assigner` to Western Electric Company, incorporated, New York, N.Y., a corporation of New York Filed Dec. 3, 1959, Ser.' N 857,087 17 Ciaiins. (Ci. 14S- 104) The present invention relates generally to magnetic cores and methods` of making the same. More particularly, the invention concerns certain improvements in the magnetic cores and methods of making them in accordance with the general principles of A. F. Bandur Patent 2,105,070, dated-January ll, 1.938, so as tol improve the physical and magnetic properties of the cores. The cores may be use'd for many purposes, but are designed primarily for use in voice frequency telephone circuits.

An object of the invention is to provide new and improved magnetic cores.

Another object or the invention is to provide new and improved methods of making magnetic cores.

More .specic objects of the invention are to increase the magnetic permeability, decrease the core loss, and increase the physical strength of magnetic cores formed by compressing magnetic particles having an insulating coating including at least one alkali metal silicate.

According to the above-noted Bandur patent, magnetic cores are produced by coating nely divided magnetic particles with an insulating composition consisting of a refractory metal silicate, magnesium V"hydroxide and an alkali metal silicate. The insulated particles are then compressed into a core and the core isiired at a temperature of 1000 to 1300" F. to -anneal the magnetic particles and cure the coating.

Another object of the invention is to provide improvements in the cores and methods of making them as generally disclosed in the Bandur patent, which improvements are directed primarily to producing cores having increased permeability, decreased core loss, or improved mechanical strength.

For many reasons, it is `desirable to increase the permeability rating of the cores as much as possible to the eX- tent that this can be done without any substantial adverse electon the core loss. For example, by increasing the permeability of the cores, it is possible to utilize cores which are smaller in size to accomplish a given result, thereby resulting in a saving both in space and in the materials used in the cores. Of particular importance from an economic standpoint, is the saving of nickel used as the predominant constituent of the magnetic particles. In the alternative, the same size core may be wound with fewer turns of Wire to obtain the same transformer action.

By decreasing the core loss, the cores functionL more eiiciently to transform voice-frequency alternating currents used in telephone transmission systems. In addi- Lion, by decreasing the core loss resultant from using a given amonntof` insulation, it is possible to utilize cores having -a smaller amount of insulation than was heretofore possible and thus to produce cores having higher permeabilities.

According to a rst embodiment of the invention, the cores are impregnated with an alkali meta-l aluminate after the compression step and before the tiring step; With this additional step, the lcorev loss is substantially lower than that" resultant without the aluminate treatment and there is no substantial adverse effect on the' permeability or other properties. Preferably, the aluminate is sodium aluminate and the impregnating step is accomplished by dipping the compressed core in an aqueous solution thereof.

3,014,825 Y Patented Dec. 26, 1961 According to a second embodiment of the invention, the insulated particles are treated with.. oil vapor prior to the compression step. The oil vapor is absorbed by and reacts with the alkali metal silicate present inthe insulation to become ,a constituent of' the insulation. With these additional steps, the permeability may be substantially increased by using higher pressures than were heretofore practical andy the core loss is substantially 11e-l duced with respect to. the core loss of cores produced by the same process but without the oil treatment.v Preferably, the insulated particles are coated with a dilute solution of a light mineral oilI dissolved in a volatile solvent therefor, and the oil-containing particles: are heated. toa temperature sucient to driveot the solvent and vaporize at least a portion of the oil.

According toa thirdV embodiment'ot` the invention, the aluminate treatment ofv the rst embodiment is; combined with the oil treatment of the, second embodiment. When both treatments are utilized, the yaluminate reacts. with the alkali metal silicate and the oil` vapor to form a' ccramic-type reaction producnwhich becomes a` constituent of the insulation. Cores producedl according to this; com bine-d process have a permeability of the samev order as that resultant from the oil treatment; alone, a core loss Iwhich is substantially lower than thatV achieved with either the aluminate or the oil treatmenty alone, and markedly superior mechanical strength.

The invention is also directed tothe magnetic cores produced in accordance with the foregoing processesfas new articles of manufacture having new and useful insulating materials separati-ng the magnetic particles. Such insulating materials, according to the various embodiments of the invention, may include (l) the heat treated reaction product of an alkali metal silicate and an lalkali metal aluminate; (2.) the heat treated reactionproduct of an alkali metal silicate and oil vapor; and (3) the heattreated, ceramic-type reaction product of an alkali metal silicate, `oil vapor, and an alkali metal aluminate.

Other objects, advantages and featuresI of the invention will appear from the following detailed description of specific embodiments and examples thereof, when taken in conjunction With the appended drawings, in which:

FIG. l illustrates graphically the typical effect of' the Y amount of insulation used on the magnetic permeability at 1800 cyclesper second in accordance ywith the prior process and Various of the improved processesl of this application; and

FIG. 2 illus-trates the corresponding effect of the amount of insulation on the core loss; at 1800 cycles per second in various cases. f

THE PROR PROCESS While various nely divided magnetic particle'spmay be' utilized in the practice' of the invention', it is preferred to use an embrittled metal alloy selected from the group of nickel `and iron alloys known as Perm'a'lloysll Of particular interest is a molybdenum'-containing Perlmalloy consisting essentially of about 82% nickel, 16% iron and 2% molybdenum. l

Such an alloy may be treated and' comminuted according to the general principles enunciated in P. Bcath et al. Patent 1,6691649, dated MayV 15, 1912-8. Accord'- ing to that patent, the'metallic constituents of the alloy are melted togetherv and oxidized in-l the molten state to embrittle the alloy. This treatment l produces a tine crystalline structure in the solidified alloy that facilitates reduction to a tiney powder by conventional' grinding and puverizing techniques.

TheV resultant magnetic powder is sieve'd through a mesh screen, and any oversize particles a're recycled; The majority oli the particles are in the range* of about 3 200 to 300 mesh. The sieved powder is next subjected to an annealing heat treatment at about 1500 to 1600 F. to remove stresses introduced into the magnetic material by the grinding operation.

The particles are then given an insulating coating comprising a refractory metal silicate, magnesium hydroxide and an alkali metal silicate. Preferably, the coating is made up of talc (a hydrated magnesium silicate), 1 part by weight; sodium silicate, 0.3 to 0.4 part by weight; and magnesium hydroxide, 0.06 to 0.08 part by weight. The optimum proportions embodied in the commercial process are talc, l part by weight; sodium silicate, 0.35 part by weight; and magnesium hydroxide, 0.07 part by weight. The sodium silicate should have a high silicate to soda ratio, preferably about 1.6 to 3.0 parts silicate to one part soda. Other refractory metal silicates, such as aluminum silicate, may be used as well as other alkali metal silicates, such as potassium silicate.

The coating composition is preferably applied in at least three stages from an aqueous suspension of the conbeen encountered with the decreases tending to occur in cases where more insulating material is used.

The drawings depict the etfect of the amount of insulation used on the magnetic properties, permeability and core loss, for a typical batch of materials. It should be understood that the absolute values will vary somewhat depending on the various other process factors (particularly the composition and particle size of the metal, the compression technique, the insulating process, and the firing schedule), but the general relationships indicated in the drawings will remain the same. Curve A in FIG. 1 depicts the magnetic permeability as a function of the amount of insulation when cores are fabricated according to the standard process described hereinbefore, while curve B in FIG. 2 illustrates the core loss according to the conventional process. Curve C in FIG. 2 illustrates the decrease in core loss achieved by the addition of the sodium aluminate treatment to the process, while the permeability resultant with the improved process is apstituents, followed by heating to dryness at a temperalture of about 270 to 300 F. after each stage as is described more fully in the Bandur patent.

After the particles have been insulated, they are cornpressed into la core of a suitable shape, such as a ring, by molding at a pressure in the neighborhood of 150,000 to 200,000 pounds per square inch. During the application of this pressure, the magnetic particles are again subjectexl to `stresses which impair the magnetic properties thereof. Therefore, the cores are again subjected to an annealing heat treatment by firing at a temperature between about 1000 and 1300 F. The cores are preferably red in a hydrogen-containing atmosphere at about 1200 F. During this heat treatment, the insulating material is fully cured.

According to the commercial embodiment of the abovedescribed process, core rings are produced having an insulating coating as described above containing about 1.25 parts by weight of the insulating material to 100 parts by weight of the magnetic powder. Such core rings have been found to exhibit an average magnetic permeability at 1800 cycles per second of 125 and an average core loss at 1800 cycles per second of 0.190 unit (ohms per unit of permeability per unit of inductance). The present manufacturing limits on such core rings have been a permeability of at least 115 and a core loss no higher than 0.240 units.

ALUMINATE TREATMENT According to the first embodiment of the invention, magnetic particles containing an insulating coating including an alkali metal silicate, and particularly particles formed generally in accordance .with the prior process just described, are impregnated with an alkali metal aluminate after the compression step and before the tiring step. With this additional step, the core loss is substantially decreased without any substantial adverse effect on the permeability. Preferably, the pressed cores are dipped for a period of 10 to 60 minutes in a 5 to 10% aqueous solution of sodium aluminate'r- While the absolute value of the core loss may vary considerably depending on the process conditions, the core loss is decreased by the aluminate treatment in all cases. The amount of insulation is preferably between vabout 0.5 and 1.5 parts by weight based on 100 parts by weight of the magnetic powder, and the process comprehends mixturesof lightly and heavily insulated powders. Over a wide range of process conditions, the improved method has been found effective to lower the core loss Abetween about 15 and 80%, with the larger decreases being realized in cases where less insulating material is used and the core loss without treatment is higher. The average permeability of cores treated according to the improved process is not substantially changed, although changes of plus or minus 5 units have proximately the same as that of the conventional process, indicated by curve A in FIG. 1.

As indicated in the drawings, maximum permeability in the typical example exists at about 0.55 part of the insulation, while the core loss decreases steadily over the range as indicated by the tapering curve B as the amount of insulation is increased. However, for other lots of powder the permeability maximum may occur between about 0.5 and about 0.8 part of the insulation depending on process conditions. From a manufacturing standpoint, it is highly desirable to use a smaller amount of insulation (0.5 to 0.9 part) than was heretofore used (1.2 to 1.3 parts) so as to achieve a permeability as near as possible to the maximum, to the extent that this can be accomplished with an acceptable core loss. As indicated in FIG. 2, curve B, the core loss is far too high when it is attempted to utilize only 0.5 to 0.9 part of the insulation by the standard method. However, because the aluminate treatment of the present invention depresses the core loss curve as indicated by curve C, it is possible to utilize more lightly insulated powder so as to achieve a higher perme ability, while maintaining the core loss within the manufacturing limit of 0.24 unit.

While the exact mechanism by which the aluminate treatment enhances the properties of the cores is not fully understood, it is believed that the aluminate solution penetrates through microscopic pores in the surface of the pressed core to impregnate the core. The sodium aluminate then reacts with the sodium silicate in the insulation to form a gel which better insulates the particles and fills any minute voids that may exist in the mass. When the vcore is later fired, the water is driven olf and there remains as a constituent of the insulation the heat treated reaction product of the sodium silicate and the sodium aluminate.

Example I According to one specific example ofthe invention, several core rings produced according to the commercial process described hereinabove were taken as a sample. These core rings were insulated with 1.25 parts by weight of the insulation as previously described. Without any intermediate treatment, a rst group of core rings was pressed from this powder at a pressure of about 160,000 pounds per square inch, and the cores were red in a hydrogencontaining atmosphere at a temperature of 1200" F. The average permeability of these core rings was and the average core loss was 0.190 unit.

A second group of cores was produced from the same powder by exactly the same process, except that these cores were soaked in a 10% aqueous solution of sodium aluminate at about 210 F. for 60 minutes after the compression step and before the lfiring step.- These cores exhibited an average permeability of 121 (a 3.2% decrease) and an average core loss of 0.160 unit, a decrease of 0.03 unit or 15.8%. While this example demonstrates the applicability of the invention to standard methods, the prin- 3,01 geen f '51 cipal advantage thereof will bek indicated in the following examples as allowing the use of a lighter coating of insnlation.

Example II In the second example, anumber of coresv were produced. with and without the aluminate' dip as described in` Example I, except that the magnetic particles were insulated with only 0.9 part by weight of the. insulation. The untreated cores; had an average permeability of 177 and a core lossof 0.60 unit (curve BV), which is. Well beyoud; the manufacturing standardof 0.24 unit. However, the treated cores exhibited a permeability ofr 174 and a core loss of only 0.19 unit (curve C), which is within the manufacturing standard. Thus, the core loss was decreased by 0.41 unit or 68% by treatment in accordance With the invention, while a relatively high permeability was achieved.

Example III In this example, a mixture of insulated powders was used to provide a high permeability, while the core rings were treated with sodium aluminate after pressing and before tiring to decrease the core loss. The insulated particles weremade up inv two distinct batches: batch A containing 0.8 part by weight of the insulation, and batch B containing the standard 1.25 parts` by weight of insulation.' These batches were otherwise produced according to the process described hereinbefore.

A series of cores was pressed from a uniform mixture of 74% of the lightly insulated particles from batch A and '26%Y of the standard particlesof batch B. Without further treatment, some of these cores were red'-, Cores produced from the mixed powders according to conventional pressing andy tiring techniques exhibited an average permeability of about 165 and a core loss of 0.28. unit.

Several cores were similarly pressed from the mixed powders, but were soaked for one hour in a solution of sodium aluminate after pressing and before firing. These cores were then tired, and the resultant permeability was 165 (no change) while the core loss was reduced to 0.176 unit. Thus, the sodium aluminate dip was eifective to produce a 37% drop inthe core loss without decreasing the permeability.

The advantages of using a mixture of lightly and heavily insulated powders is that the resultant permeability tends to approximate the weighted' average of the two constituents, while the core loss is much closer to that of the heavily insulated powder than the average figure. Preferably, between 70 and 80% of particles having 0.50 to 0.90 part insulation per 100 parts. of the metal. are mixed with 30 to 20% particles having 1.2 to 1.5 partsof insulation.

OIL TREATMENT According tothe second embodiment of the invention, magnetic particles containing an insulating coating includ'- ing an alkali metal silicate, and particularly particles formed generally in accordance with the prior process described hereinbefore, are treated with oil vapor prior to the compression step. Preferably, the insulated particles are coated with a thin tilm of oil, and are then heated to vaporize at least a portion of the oil. The oil vapor is absorbed by and reacts with the alkali metal silicate to become a constituent of the insulation. With this additional step, the permeability ofthe finished core lmay beV substantially increased by using higher pressures than were heretofore practical and the core lossr is substantially decreased in any case.

Preferably, between 0.05 and 0.35 part by weight, based on` 100 parts by weightv of theinsulated powder, of a light. mineral oil is. added by mixing. the particles with a dilute solution of the` oil in a. volatile solvent therefor. The dilute solution is used; so that. the small amount. ofy oil used, preferably about 0.2 part by Weight, is extended to cover substantially all of the insulated particles with an exceedingly thin iilm of oil. This iilm of oil may have a thickness of approximately 0.001 mill. The dilute solution` of oil` may range between. about 1 and about 3.5% oil, with the remainder'being solvent. It isv also preferred to add a surface-active agent,` such as -Aerosol (dihexylsodiumsulfosuccinate.) or Levapon,v to the oil-solvent mixture so as. to. lower the: surface tension thereof and facilitate. coating of the particles with a thin film of oil.` Approximately 4 drops. of Aerosol are suflicient touse with 400 milliliters of th oil-solvent solution..

While the vapors of. substantially all oils` are absorbed by sodium silicate and may be utilized, onev highly suitable oil is a nonblended unreclaimed straight run. oil (a light mineral oil.) having the following. properties: specific gravity, 0.887; viscosity at 11009 E. (S.S.U.), 10G-1351; flash paint 310 F.; boiling point., 250 F.; volatile matter 0.35% maximum at 212 F.; neutralization No., 0.10 milligram of KOH- t'oneutralize 1 gram of oil; and. ash, 0.01% maximum.

The particular vsolvent is not critical; however, the

solvent must be capable of dissolving the required amount of the oil, up to about 3.5%, and must be vaporized at'a temperature substantially below that where the oil begins to vaporize. Acetone, which has a low. viscosity and vaporizes'at about 134"v F.,.is suitable for application with ay wide variety of oils. Other' suitable solvents include methyl. chloroform, carbon tetrachloride and toluol.

'Ille amount of oil used is dependentA on the. amount of sodium silicate in the insulation, and the more` heavily insulated particles are capable of absorbing a larger amount of oil since more sodium silicate is present..

The oil-coated particles should be heated. to atemperature at which the oil begins to vaporize, preferably to about 260 F. for the specitic oil described which boils at about 250 F. This temperature should be-maintained for a suicient time, preferably about 5 to 15 minutes, to vaporize a substantial portion of ther oil, at leastl a portion of the oilv vapor being absorbed. by the sodium silicate. When the core is later tired, any remaining oil is vaporized and the heat-treated reaction product of the sodium silicate and the oil vapor remains as a. con.- stituent of the insulation. j

'A representation` of the: permeability increase achieved by the oil treatment combined with an increased pressure is indicated by curve'D, FIG. 1, while an indication of the decrease in core loss is given in curve. E, FIG. 2'. It will. be appreciated that the oil treatment. permits the production of high permeability cores. (up to'. about. 240

to 250) for two reasons: rst, this treatment permits the use of a higher pressure than was heretofore possible, which results. in a denser core having a higher permeability; and second, by drastically reducing the core loss as. indicated. by curve E in FIG. 2,. the method allows the use. of a smaller amount of insulation, between about 0.5 v and 1.0 part. byweight, tov take advantage of they phenomenon of maximum permeability. s

Example IV'y tion tends to: be broken down and the resultV is an exi cessive core. loss;

A second groupv of cores' was made. from the same powder in the same mannenexceptthat the pressure was approximatelyl 200,000. pounds per squarev inch. and the insulated particles were coated with a 2.5% solutionI of the light mineral oil described hereinbefore dissolved in acetone and containing a minor amount of AerosoL ApproximatelyS milliliters of the solution were used for 100 grams of the insulated powder, sufficient to yield 0.2 part by weight of the oil per 100y parts by weight of the insulated powder. The oil-containing mixture was then heated to a temperature of 260 F. for about 10 minutes to drive off the solvent and vaporize a substantial portion of the oil. These oil-coated particles were then compressed into cores and ired according to the standard procedure. The cores thus formed exhibit an average permeability of 217 (curve D) and an average core loss of 0.150 unit (curve E), which is well within the manufacturing limit. Thus, by the improved treatment, the permeability was raised 40 units or 22.6%, while the core loss was lowered 0.45 unit or 75%.

Example V In this example, a series of cores was produced as described in Example lV, except that only 0.7 part by weight of the insulation was utilized. This percentage of insulation is designed to achieve a near maximum permeability, as indicated in FG. l (curve D). Without the oil treatment, these cores had a permeability of 210 (curve A) and a core loss of 1.05 units (curve B). The cores treated with oil, however, showed a permeability of 233 (curve D) and a core loss of only v0.175 unit (curve E).-

CO-l/lBlNE-D METHOD According to the third embodiment of the invention, the aluminate treatment described hereinbefore is combinedwith the oil treatment just described to produce further improvements in the physical and magnetic properties of the cores. Cores produced according to the combined method have a permeability about the same as that realized by the oil treatment alone (curve D, FIG. l), a core loss lower than that produced with either the aluminate treatment or the oil treatment alone (curve F, FIG. 2), and markedly improved physical strength and resistance to breaking.V

When these treatments are vcombined and the cores are tired, a ceramic-type reaction product is formed between the sodium silicate,-the oil vapor absorbed thereby, and the sodium aluminate. This reaction product becomes an important constituent of the insulation, and apparently contributes greatly to the improved physical strength of the cores.

Example AVI A number of the cores containing 0.9 part by Weight of insulation were given an loil treatment as described in Example IV and were also given an aluminate treatment as described in Example II. The cores treated according to this combined process had a permeability of 215 and a core loss of 0.115 unit, as compared with 217 and 0.150 for the oil treatment alone, 174 and 0.19 for the aluminate treatment alone, and 177 and 0.60 without any of these treatments.

The cores manufactured in accordance with the combined process were substantially harder and more difficult to break than those produced by any of the other methods, probably due to the presence of the ceramictype constituent in the insulation. When it was attempted to scratch a broken cross section of a core produced in accordance with the combined method with a sample of Permalloy metal, the insulated particles did not crumble; instead, the metal rubbed ott on the surface of the insulated particles. However, when a similar test was applied to cores made by the prior process or those made with the aluminate or oil treatments alone, the powder crumbled in response to scratching by metallic Permalloy. In addition, the cores produced by the combined process -were much more diicult to fracture than any of the other cores.

Example VII A number of cores were produced as described in Example V with 0.7 part by weight of the insulation, except that the sodium aluminate treatment as in Example II was added to the oil treatment. These cores displayed a permeability of 228 and a core loss of 0.125 unit, as compared to a permeability of 233 and a core loss of 0.175'resultant with the oil treatment alone. These cores were also substantially harder and more resistant to breakage than the cores treated with oil alone.

As described in my vcopending application Serial No. 857,104, tiled contemporaneously herewith, all of the core rings formed in accordance with the present invention may be soaked in water after the firing operation and then retired in a hydrogen atmosphere at about 1200 F. so as to increase the permeability by about 20 additional units without any substantial change in the core loss.

The foregoing examples illustrate the application ofk the various embodiments of the invention to produce improved magnet cores. While various specic examples of the invention have been described in detail hereinabove, it will be obvious that various modifications may be made from the specific details described without departing from the spirit and scope of the invention.

What is claimed is:

l. In a process of making a magnetic core wherein iinely divided magnetic particles are given an insulating coating including an alkali metal silicate, wherein the insulated particles are compressed into a core, and wherein the core is tired to anneal the magnetic particles and cure the coating; the improved method which comprises vthe additional step-of impregnating the core with an alkali metal aluminate after the compression step and before the firing step.

2. In a process of making a magnetic core wherein nely divided magnetic particles are given an insulating coating comprising a refractory metal silicate, magnesium hydroxide, and an alkali metal silicate; wherein the insulated particles are compressed into a core; and wherein the core is tired at a temperature of 1000 to 1300 F.: the improved method which comprises the additional step of dipping the compressed core in an aqueous solutionof sodium aluminate after the compression step and before the ring step.

3. In a process of making a magnetic core wherein finely divided magnetic metal particles are given an insulating coating of between 0.5 and 1.5 parts by weight, based on parts of the metal, of a composition consisting essentially of talc, 1 part by weight, sodium silicate, 0.3 to 0.4 part by weight, and magnesium hydroxide, 0.06 to 0.08 part by weight; wherein the insulated particles are then compressed into a core; and wherein the core is red in a hydrogen-containing atmosphere at a temperature of about l200 F.: the improved method which comprises the additional step of dipping the core in a 5 to 10% aqueous solution of sodium aluminate for a period of l0 to 60 minutes after the compression step and before the firing step.

4. The method in accordance with claim 3 wherein the insulated powder is prepared in two distinct batches, a iirst containing between 0.50 and 0.90 part by weight of the insulation per 100 parts of the metal and the second containing between 1.2 and 1.5 parts Vby weight of the insulation per 100 parts of the metal; and wherein a uniform mixture of 70 to 80% particles from the irst batch with 30 to 20% particles `from the second batch is compressed into the core.

5. In a process of making a magnetic core wherein iinely divided magnetic particles are given an insulating coating including an alkali metal silicate, and Iwherein the insulated particles are compressed into a core; the improved method which comprises the additional step of treating the insulated particles with oil vapor prior to the compression step, the oil vapor being absorbed by and antenas reacting with the alkali metal silicate to become a constituent of the insulation.

6. In a process of making a magnetic core wherein iinely divided magnetic particles are given an insulating coating including an alkali metal silicate, and wherein the insulated particles are compressed into a core: the improved method which comprises the additional steps of coating the insulated particles with a thin lm of oil prior to the compression step; and heating the oil-containing particles prior to the compression step to vaporize at least a portion of the oil, the oil vapor being absorbed by and reacting wtih the alkali metal silicate to become a constituent of the insulation.

7. In a process of making a magnetic core wherein finely divided magnetic particles are given an insulating coating comprising a refractory metal silicate, magnesium hydroxide, and an alkalimetal silicate; wherein the insulated particles are compressed into a core; and wherein the core is red to anneal the magnetic particles and cure the coating: the improved method which comprises the additional steps of coating the insulated particles prior to the compression step with between 0.05 and 0.35 part by weight, based in 100 parts by weight of the insulated powder, of an oil by mixing the particles with a dilute solution of the oil in a volatile solvent therefor; and heating the oilcontaining particles prior to the compression step to a temperature suiiicient to drive off the solvent and vaporize at least a portion of the oil, the oil vapor being absorbed by and reacting with the alkali metal silicate to become a constituent of the insulation.

8. In a process of making a magnetic core wherein iinely divided magnetic metal particles are given an insulating coating of between 0.5 and 1.0 part by weight, based on 100 parts of the metal, of a composition consisting essentially of talc, 1 part by weight, sodium silicate, 0.3 to 0.4 part by weight, and magnesium hydroxide, 0.06 to 0.08 part by weight; wherein the insulated particles are then compressed into a core; and wherein the core is tired at a temperature of 1000 to l300 F.: the improved method which comprises the additional steps of coating the insulated particles prior to the compression step with between 0.05 and 0.35 part by weight, based on 100 parts by weight of the insulated powder, of a light mineral oil by mixing the particles with a 1 to 3.5% solution of the oil dissolved in acetone and containing a minor amount of a surface active agent; and heating the oil-containing particles prior to the compression step to a temperature between 5 and 15 F. above the temperature at which the oil begins to vaporize to drive off the acetone and vaporize a portion of the oil, the oil vapor being absorbed by and reacting with the sodium silicate to become a constituent of the insulation.

9. In a process of making a magnetic core wherein finely divided magnetic particles are given an insulating coating including an alkali metal silicate, wherein the insulated particles are compressed into a core, and wherein the core is red to anneal the magnetic particles and cure the coating: the improved method which comprises the additional steps of treating the insulated particles with oil vapor prior to the compression step, the oil vapor being absorbed by and reacting with the alkali metal silicate; and impregnating the core with an alkali metal aluminate after the compression step and before the tiring step, the aluminate reacting with the silicate and the oil vapor upon firing to form a ceramic-type reaction product which becomes a constituent of the insulation.

l0. In a process of making a magnetic core wherein finely divided magnetic particles are given an insulating coating comprising a refractory metal silicate, magnesium hydroxide, and an alkali metal silicate; wherein the insulated particles are compressed into a core; and'wherein the core is ired at a temperature of 1000 to 1300 F.: the improved method which comprises the additional steps of coating the insulated particles with a thin film of a light mineral oil prior to the compression step by mixing the particles with a dilute solution of the oil in a volatile solvent therefor; heating the oil-containing particles prior to the compression step to a temperature sufiicient to drive oit the solvent and vaporize at least a portion of the oil, the oil vapor being absorbed by and reacting with the alkali metal silicate; and dipping the compressed core in an aqueous solution of sodium aluminate after the compression step and before the firing step, the sodium aluminate reacting with the alkali metal silicate and the oil vapor upon ring to form a ceramic-type reaction product which becomes a constituent of the insulation.

11. In a process of making a magnetic core wherein finely divided magnetic metal particles are given an insulatingcoating of between 0.5 and 1.0 part by weight, based on parts of the metal, of a composition consisting essentially of talc, 1 part by weight, sodium silicate,

0.3 to 0.4 part by weight, andmagnesium hydroxide,A

0.06 to 0.08 part by weight; wherein the insulated particles are then compressed intoa core; and wherein the coreV is tired at a temperature of 1000 to 1300 F.: the improved method which comprises the additional steps of coating the insulated particles prior to the compression step with between 0.05 and 0.35 part by weight, based on 100 par-ts by weight of the insulated powder, of a light mineral oil by mixing the particles with a 1 to 3.5% solution of the oil dissolved in acetone and containing a minor amount of a surface active agent; heating the coated particles prior to the compression step to a temperature about 5 to 15 F. above the temperature at which the oil begins to vaporize to drive off the acetone and vaporize a portion of the oil, the oil vapor being absorbed by and reacting with the sodium silicate; and dipping the core in a 5 to 10% aqueous solution of sodium aluminate for a period of 10 to 60 minutes after the compression step and before the tiring step, the sodium aluminate reacting with the sodium silicate and the oil vapor upon tiring to form a ceramic-type reaction product which becomes a constituent of the insulation,

12. As a new article of manufacture, a magnetic core composed of particles of a magnetic material separated by an insulating material comprising the heat treated reaction product of an alkali metal silicate and an alkali metal aluminate.

13. As a new article of manufacture, a magnetic core composed of particles of a magnetic material separated by an insulating material comprising the reaction product of sodium silicate and sodium aluminate when heat treated at a temperature of 1000 to 1300 F.

14. As a new article of manufacture, a magnetic core composed of particles lot a magnetic material separated by an insulating material comprising the heat treated reaction product of an alkali metal silicate and oil vapor.

15. As a new article of manufacture, a magnetic core composed of particles of a magnetic material separated by an insulating material comprising the reaction product of sodium silicate and the vapor of a light mineral oil when heat treated at al temperature of 1000 to 1300 F.

16. As a new article of manufacture, a magnetic core composed of particles of a magnetic material separated by an insulating material comprising the heat treated, ceramic-type reaction product of an alkali metal silicate, oil vapor, and an alkali metal aluminate.

17. As a new article of manufacture, a magnetic core composed of particles of a magnetic material separated by an insulating material comprising the ceramic-type reaction product of sodium silicate, having the vapor of a 'light mineral oil absorbed thereby, and sodium aluminate when heat treated at a temperature of 1000 to 1300" F.

References Cited in the tile of this patent UNITED STATESr PATENTS 2,531,445 Laylock Nov. 28, 1950 2,744,040 Altmann May 1, 1956 2,827,384 Freyhold Mar. 18, 1958 2,919,996 Teja Ian. 5, 1960 

1. IN A PROCESS OF MAKING A MAGNETIC CORE WHEREIN FINELY DIVIDED MAGNETIC PARTICLES ARE GIVEN AN INSULATING COATING INCLUDING AN ALKALI METAL SILICATE, WHEREIN THE INSULATED PARTICLES ARE COMPRESSED INTO A CORE, AND WHEREIN THE CORE IS FIRED TO ANNEAL THE MAGNETIC PARTICLES AND CURE THE COATING; THE IMPROVED METHOD WHICH COMPRISES THE ADDITIONAL STEP OF IMPREGNATING THE CORE WITH AN ALKALI 